(ii) The realization of the negative impedance in the GPIN-TTD and the amplification of the THz radiation
passing its plane, associated with the TT resonances, require low or moderate values of the THz dielectric constants of the substrate and the top/passivating layer. Among such materials one can mention
crystalline hBN as a material for the substrate with a thin passivation layer (\(\kappa _{eff} \simeq 3\) ) [1]}
as well as polyimide, porous BN/polyimide composite, and amorphous HBN (\(\kappa \simeq 1.16\) ) [2]}, [3]}.
It is interesting that the GPIN-TTDs on some of such substrates can be used for flexible THz sources (see [4]}).
